Cellular responses to DNA damage and other stresses are important determinants of cell viability and mutagenesis and impact the development of a wide range of human diseases. DNA damage responses, in particular, are major determinants of both cancer development and outcomes of cancer therapies. Induction of signal transduction pathways is a critical aspect of cellular responses to these stresses and significant advances have been made in recent years elucidating the biochemical steps in such signaling pathways. Clarification of such steps enables modulation of these responses, which can enhance research studies and can lead to the generation of new medicines to prevent and treat these diseases. This application describes an enhancement of a novel technique developed in the Kastan laboratory in which DNA double strand breaks (DSBs) are introduced at defined sites in the human genome. Introduction of site-directed DNA damage permits studies of molecular events occurring at and around the DSB and direct assessment of repair (re- ligation) of DNA breaks. The enhancements of this technique permit greater temporal control of introduction of the DSBs and allow assessment of the kinetics and completeness of DSB repair. This enhanced approach will be used in the experiments proposed in this application to elucidate the dynamic changes in protein movements/modifications occurring at and around the DSB and explore factors which may determine efficiency of repair of the DNA breaks. One major focus is exploration of the molecular controls of nucleosome disruption at the DSB site and elucidating the functional importance of this nucleosomal disruption. It is well established that numerous modifications occur in chromatin-associated proteins surrounding DSBs during the damage/repair process;however, the functional role(s) of many of these modifications, especially related to DNA repair, remain to be clarified. Experiments are proposed that will use this novel system to explore the potential impact of these chromatin changes surrounding the DSBs on the ability of the cell to repair (re-ligate) DNA breaks. Further, the mechanisms and impact of the breast cancer susceptibility gene product, Brca1, on this process will be investigated. The proposed studies will provide novel insights into molecular mechanisms associated with the DNA DSB repair process in human cells.